400MW Generator pulling too many MVARS (lagging power factor)

[GamecockEngr]

We have checked the voltage meters used to sync and they are good as are the PT's used to read the voltage. We just installed a new excitation system. The generator terminal voltage is controlling to the proper set point and the dc is balanced perfectly. Here is the problem...when we tie on the MVARS are running ~60-80 MVARS at almost no load (5-10 MW). Usually we run 0-10 MVARS at 0-10 MW. The power factor is .1-.3

What could be causing this in the excitation system?

I am a dumb flange head (mechanical engineer). I need help with this stuff!

 

[davidbeach]

MVArs in or MVArs out? In would indicate under excitation while out would indicate over excitation. As to cause, well there isn't nearly enough information available, other than to suggest that the new excitation system wasn't commissioned properly.

 

[GamecockEngr]

I can give you as much info as you need. MVARS OUT.

I am hesitant to give too much specifics because in the off chance someone reads this I dont want to get in any hot water if we have a warranty claim on the OEM.

 

[davidbeach]

What operating mode? Voltage control? Watt/VAr control? Power factor control?

 

[GamecockEngr]

Mode is voltage control. 22kv generator terminal. 230kv high side. Metering Pt's are good as are meters. You think they are raising field current too high. Field (rotor) and stator are newly re-wound.

 

[GTstartup]

Why don't you just reduce the voltage setpoint until you get the VARs you want?

Plus check you are not actually in Field Current control.

 

[GTstartup]

Also check your old AVR settings against your new settings, especially any static droop or actual value voltage bias (compensatation for PT inaccuracies)

 

[GTstartup]

Actually, since this is a new AVR you should be asking whoever installed and commissioned it

 

[dpc]

I would verify you are actually in automatic mode of some kind. If in voltage control, you can try lowering the voltage setpoint - the vars should come down.

You could also try putting the AVR in manual and try adjusting the field voltage to see if you have any control at all.

I suspect the AVR is not getting the correct sensing voltage - could be a line-neutral voltage when it is expecting a line-to-line. A 400 MW generator should be able to move the system voltage a bit.

I'd walk back through the PT connections, PT ratios, and AVR configuration.

 

[ScottyUK]

Have you checked the simple things, like is the GSU transformer's OLTC at the same tap position as it was prior to the upgrade? Three or four taps would probably give you 80 MVAr on a unit that size.


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If we learn from our mistakes I'm getting a great education!

 

[nawao]

Invert the polarity of the droop CT and see how is the behaviour

 

[bartb102]

If this is a parallelling generator then why not operate in PF mode? In AVR mode if voltage setting is higher than utility voltage excitation will increase trying to raise terminal voltage into an infinite load, increasing vars output. Generator output power (KW) is controlled by energy source (turbine, diesel, etc.) only, not load.

 

[ScottyUK]

Utility machines are usually despatched separately on load and reactive load: PF control ties them together, so a change in MW also requires adjustment to the AVR. One more job than there needs to be!


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If we learn from our mistakes I'm getting a great education!

 

[GTstartup]

One thing is not clear. The OP stated:

"when we tie on the MVARS are running ~60-80 MVARS at almost no load (5-10 MW). "

Is this immediately on synchronizing? If yes, then it's not an AVR issue but a synchronizer issue.

 

[m3ntosan]

Synchronizing with big differences in voltage magnitude a large MVAR power flow will appear across the CB as it is closed.

May you grow up to be righteous, may you grow up to be true...

 

[dpc]

Synchronizing with big differences in voltage magnitude a large MVAR power flow will appear across the CB as it is closed.

We know.




"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[waross]

I would say that 60-80 MVARs at a load of 5-10 MW is overexcitation. This will primarily be caused by the AVR. But a synchronizer issue may be causing the AVR to overexcite.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

Lots of good comments and questions above. One other thing, the vars at a given voltage depends not only on generator and it's control, but on the grid system .... which may have changed.

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[electricpete]

Clarification in bold:

...One other thing, the vars at a given voltage depends not only on generator and it's control, but on the grid operating conditions .... which may have changed.

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[esee135]

Where is your generator breaker (high side or low side of the step up transformer)? Also, are you sync PT's accounting for the tap position.

As other have indicated, drop your voltage and the VARs will follow. If your unit has picked up VARs after a long period post syncing, your bus conditions have changed and the unit is boosting VARs to attain the voltage set-point. This is normal.